Rechargeable large-capacity batteries are commonly used in marine systems, alternative energy systems, recreational vehicle (RV) systems, industrial lift truck applications and electric vehicles. These applications typically use the so-called deep-cycle type, such as wet-cell and gel-cell batteries. Since these applications depend upon battery power for vital systems (such as locomotion), knowing the actual state-of-charge of a battery is very useful. Otherwise, if the battery's charge is depleted without sufficient warning, a user may be stranded and unable to reach a power source to recharge the battery.
Accurate monitoring of a battery's state-of-charge is significant for other reasons. A rechargeable battery may be damaged by excessive discharge or by under-charging. Accurate monitoring allows the battery to be recharged before the state-of-charge is excessively low and avoid under-charging, thereby increasing the life of the battery.
Among the methods used by existing battery monitors are a) detecting the specific gravity of the battery electrolyte; b) measuring the terminal voltages of the battery; and c) measuring and tracking over time the charge drawn from and supplied to the battery. However, none of these battery monitoring methods satisfactorily compensate for conditions that affect the actual battery capacity. In particular, such conditions include discharging/charging currents and changing ambient temperatures.
Specifically, the latter of the above-listed monitors is a relatively inexpensive type of battery monitor. These monitors require a baseline charge capacity of the battery's to be selected by a user and/or be experimentally determined. These types of monitors track the quantity of charge removed from and added to the battery relative to the baseline charge capacity of the battery. In practice, such monitors, however, typically are not sufficiently accurate for applications such as marine vessels and electric vehicles.
These monitors are not accurate because conditions differing from those which produced the baseline charge capacity alter the actual charge capacity of the battery. Again, these conditions include the discharging/charging current and the ambient temperature around the battery. Nonetheless, in many practical applications of large-capacity batteries, such as those listed above, the battery is typically subject to varying current loads (or charging currents) and changing temperature conditions.